This invention relates in general to algorithms for anti-lock brake systems and in particular to a control algorithm which provides improved control of wheel brake pressure on speed sensitive road surfaces.
Braking a vehicle in a controlled manner under adverse weather conditions, such as rain, snow or ice, generally requires precise application of the vehicle wheel brakes by the vehicle operator. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure which causes the vehicle wheels to lock-up such that excessive slippage between the wheels and the road surface takes place. Wheel lock-up conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout. Accordingly, an Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of the vehicle wheel brakes to prevent lock-up of the associated vehicle wheels. One type of ABS controls only the rear vehicle wheel brakes. Such a system is referred to as a RWAL in the following description.
A typical prior art RWAL system 10 is illustrated in FIG. 1. The RWAL system 10 includes a normally open solenoid valve 22 connected between the vehicle master cylinder 14 and the controlled rear wheel brakes 20a and 20b. When actuated, the normally open solenoid valve 22 closes to isolate the rear wheel brakes 20a and 20b from the master cylinder 14. Accordingly, the normally open solenoid valve 22 is referred to below as an isolation valve. The isolation valve 22 also can be selectively opened to increase the pressure at the rear wheel brakes 20a and 20b. The RWAL system 10 also includes a normally closed solenoid valve 26, which is referred to below as a dump valve. The dump valve 26 is selectively opened to reduce the pressure at the rear wheel brakes by bleeding brake fluid from the rear wheel brakes 20a and 20b to an accumulator 28. The isolation and dump valves 22 and 26 are mounted within a control valve 21.
The vehicle brake system master cylinder 14 provides a source of pressurized hydraulic brake fluid to the RWAL system 10. Thus, a separate hydraulic source, such as a motor driven pump, which is usually included in a four wheel ABS, is not needed. This reduces the complexity and cost of manufacturing the RWAL system 10, which is typically referred to as a passive system. The RWAL system 10 further includes an electronic control module 30 which is electrically connected to a wheel speed sensor 40 and to the isolation and dump valves 22 and 26. The control module 30 can be mounted directly upon the control valve 21 or located remotely therefrom.
The control module 30 includes a microprocessor (not shown) which is programmed to control the RWAL system in accordance with a control algorithm and parameters permanently stored in a Read Only Memory (ROM). Typically, the control algorithm is trimmed for the particular vehicle in which the ABS is installed. The microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL system 10 illustrated in FIG. 1 is included in U.S. Pat. Nos. 4,790,607 and 4,886,322.
During vehicle operation, the microprocessor in the ABS electronic control module 30 continuously receives speed signals from the wheel speed sensor 40. During a vehicle braking cycle, the ABS microprocessor monitors the rear wheel speed and deceleration. The microprocessor calculates a theoretical speed ramp, which represents the speed the vehicle would travel if decelerated at a predetermined maximum rate, such as, for example, 1.0 g. The microprocessor compares the actual rear wheel speed to the theoretical ramp. If the rear wheel deceleration reaches a predetermined value, such as, for example, 1.3 g, the microprocessor determines that the rear wheel brakes 20a and 20b may be approaching a rear wheel lock-up condition. Accordingly, the ABS microprocessor closes the isolation valve 22 to isolate the rear wheel brakes 20a and 20b from the master cylinder 14. If the rear wheel speed departs form the theoretical ramp in addition to, or in place of, the deceleration condition, the ABS microprocessor determines that the rear wheel brakes 20a and 20b are certainly approaching a lock-up condition and the microprocessor maintains the isolation valve 22 in the closed position. The ABS microprocessor then selectively opens the dump valve 26 to reduce the pressure applied to the rear wheel brakes 20a and 20b to correct the rear wheel speed departure. Once the wheel speed departure has been corrected and the controlled wheel has spun up to the vehicle speed, the microprocessor opens the isolation valve to initiate a second wheel speed departure to adjust the rear wheel brake pressure upward.
The operation of the RWAL system is illustrated by the graphs shown in FIG. 2. The upper curve shows the rear wheel speed as a function of time while the lower curve shows the rear wheel brake pressure as a function of time. The middle curves illustrate the operation of the isolation and dump valves 22 and 26 as a function of time. The solid curve labeled 60 represents the velocity of the rear wheels while the dashed curve labeled 64 represents the vehicle velocity. The first and second wheel speed departures are labeled 60a and 60b, respectively. Following correction of the second wheel speed departure, which occurs at time t.sub.7, the rear wheel brake pressure is maintained a constant level P.sub.e, as shown in the lower curve.
If the vehicle transitions from a low mu to a high mu road surface, a key feature included in the algorithm utilized by the RWAL system 10 is a corresponding increase in the braking effort exerted by the rear wheel brakes 20a and 20b to utilize the increased mu. An example of such a transition is shown at t.sub.8 in FIG. 2. The road surface transition can be detected when the microprocessor detects an increased vehicle deceleration due to the greater braking effect of the uncontrolled front wheel brakes 19a and 19b upon the higher mu road surface. Typically, if the vehicle deceleration increases by a predetermined amount, such as 0.25 g, from a vehicle deceleration value measured near the beginning of the stop, it is known to reopen the isolation valve 22 to generate an unlimited series of reapply pulses 62b. The resulting increased pressure to the rear wheel brakes 20a and 20b initiates a third wheel speed departure, which is labeled 60c in FIG. 2. At time t.sub.10, a dump pulse 63d is generated to open the dump valve 26 to reduce the rear wheel brake pressure to a level P.sub.g to correct the third rear wheel departure. Thereafter, the rear wheel brake pressure is held at the level P.sub.g, which is greater than the previously held level P.sub.e.